46 LAND AND FRESH-WATER MOLLUSKS. 
the whorls; and transverse when they coincide 
with the lines of growth. The shell is secreted 
by the mantle, and consists of layers of mem- 
brane strengthened by calcareous matter, and 
does not exhibit such an amount of diversity as 
that of the bivalve shells. The colours which 
are diffused over the shell are due to the secre- 
tion from the mantle, of pigments, which are 
impregnated chiefly in the epidermal matter. 
The aperture of the shell is accurately closed by 
a horny plate attached to the back of the foot, and 
is called the operculum (q); it is calcareous in some 
other snails. As regards its mode of growth, it 
is concentric, that is, increasing equally all round ; 
in the genus Valvata it is spiral, growimg only 
on one edge, and revolving as it grows. 
Animal.—The body of the animal is enclosed 
in the mantle, which is open in front, and through 
which the head and foot project; the abdomen, 
containing the mass of the viscera, is wound 
upon itself in the form of the shell. The mantle in 
its natural position covers the back and forms a 
large fold or cavity, on the left side of which lie 
the gills, and into which the terminations of the 
digestive, reproductive, and urinary organs open. 
When the snail (see fig. 5) is protruded from 
its shell, the following parts may be seen :— 
The Foot, by which it makes its way in the 
world, is avery broad, flat, expanded disk, and is 
